1. Field of the Invention
The present invention relates to an image forming apparatus having a fixing rotor for fixing onto a recording medium while carrying it therebetween, and application means for applying a release agent onto the fixing rotor.
2. Related Background Art
Conventionally, it has been widely practiced in image forming apparatuses such as an electrophotographic apparatus or an electrostatic recording apparatus that an unfixed toner image is fixed by a heat roller fixing device.
It is the common practice that a release agent is applied onto the heat roller in the heat roller fixing device to prevent offset of toner.
FIG. 1 illustrates an example of an image forming apparatus having a heat roller fixing unit. This example is a full-color image forming apparatus of electrophotographic type.
A color electrophotographic apparatus as illustrated in FIG. 1 includes a transfer medium conveyance system I provided from the right side of apparatus main body 1 (as seen in FIG. 1) to the substantially central portion thereof, a latent image forming unit II provided in the substantially central portion of apparatus main body 1 in proximity to a transfer drum 15 constituting the transfer medium conveyance system I, and developing means disposed in proximity to the latent image forming unit II, namely, a rotary developing unit III.
The transfer medium conveyance system has a constitution as described below. First, an opening is formed on a right wall of the apparatus main body 1 (as seen in FIG. 1), and detachable transfer medium supply trays 2, 3 are disposed in the opening, part of the trays projecting from the apparatus. Sheet supply rollers 4, 5 are arranged substantially just above the trays 2, 3. Sheet feed rollers 6 and sheet supply guides 7, 8 are provided so as to communicate from the sheet supply rollers 4, 5 to a transfer drum 15 that is transfer means disposed leftward and rotatable in a direction of the arrow A. Around the external curved surface of the transfer drum 15 are disposed successively an abutting roller 9, a gripper 10, a transfer medium separation electrostatic charger 11, and a separation claw 12, from the upstream side in a rotational direction of the drum 15 to the downstream side. Also, on the inner side of the transfer drum 15 are disposed a transfer electrostatic charger 13 and a transfer medium separation electrostatic charger 14. The transfer drum has a transfer sheet (not shown) composed of polyvinylidene fluoride adhered on a portion of the transfer drum around which the transfer medium is wound, so that the transfer medium is electrostatically adhered intimately onto the transfer sheet. Conveying belt means 16 is disposed right upward of the transfer drum 15 and in proximity to the separation claw 12, and a fixing unit 18 is disposed in the terminal (right) end portion of the conveying belt means 16 in a direction of conveying the transfer medium. An exhaust tray 17 extending from the apparatus main body 1 and detachable therefrom is disposed further downward of the fixing unit 18 in the conveying direction.
Next, the constitution of latent image forming unit II will be described below. First, a photosensitive drum 19 which is a latent image carrier and is rotatable in a direction of the arrow B in FIG. 1, is disposed with its external curved surface contacting the external curved surface of the transfer drum 15. Around the external curved surface of the photosensitive drum 19 are disposed successively a static eliminator 20, cleaning means 21 and a primary electrostatic charger 23, from the upstream side in a rotational direction of the photosensitive drum 19 to the downstream side. Further, image exposure means 24 such as a laser beam scanner for forming an electrostatic latent image and image exposure reflecting means 25 such as a mirror are arranged upward of the external curved surface of the photosensitive drum 19.
Finally, the constitution of rotary developing device III is as follows. A rotatable housing 26 (hereinafter referred to as a "rotor") is disposed opposed to the external curved surface of the photosensitive drum 19, including four types of developing units mounted at four positions along a circumferential direction thereof to visualize (or develop) an electrostatic latent image formed on the external curved surface of the photosensitive drum 19. The above four types of developing units involve a yellow developing unit 27Y, a magenta developing unit 27M, a cyan developing unit 27C and a black developing unit 27Bk.
The overall sequence of the image forming apparatus having the above constitution will be first described briefly in an instance of full-color mode. If the photosensitive drum 19 is rotated in a direction of the arrow B in FIG. 1, a photosensitive member on the photosensitive drum 19 is evenly charged by the primary electrostatic charger 23. The apparatus of FIG. 1 has an operation speed (hereinafter referred to as "process speed") of 160 mm/sec. If the photosensitive member is evenly charged by the primary electrostatic charger 23, an image is exposed to a laser beam E modulated by a yellow image signal of original 28, so that an electrostatic latent image is formed on the photosensitive drum 19 and developed by the yellow developing unit 27Y prepositioned at a developing position by rotation of the rotor 26.
On the other hand, the transfer medium conveyed via the sheet feed guide 7, sheet feed rollers 6 and sheet feed guide 8 is held by the gripper 10 at a predetermined timing, and wound electrostatically around the transfer drum 15 by virtue of the abutting roller 9 and an electrode placed opposite to the abutting roller 9. The transfer drum 15 is rotated in synchronism with the photosensitive drum 19 in a direction of the arrow A in FIG. 1, visible image developed by the yellow developing unit 27Y is transferred by transfer electrostatic charger 13 at a position where the external curved surface of the photosensitive drum 19 and the external curved surface of the transfer drum 15 are in contact with each other. The transfer drum 15 is continuously rotated, and placed in a ready state for the transfer of the next color (magenta in FIG. 1).
On the other hand, the photosensitive drum 19 has its static charges eliminated by the static eliminator 20, and is cleaned by cleaning means 21 relying on a conventionally well-known blade method, thereafter, is charged again by the primary electrostatic charger 23, and subjected to image exposure as described above upon accepting a next magenta image signal. The above rotary developing device is rotated while an electrostatic latent image with the magenta image signal is being formed on the photosensitive drum 19 by image exposure, so that the magenta developing unit 27M is positioned at the developing position predetermined as above for effecting a predetermined magenta development. Subsequently, the same process as described above is performed for each color of cyan and black. When the transfer of four colors is completed, the transfer medium has a four-color visible image formed thereon. Then, the transfer medium is static eliminated by the eliminator 20 and the charger 14, released from the gripping with the gripper 10, separated from the transfer drum 15 by virtue of the separation claw 12, conveyed to the fixing unit 18 on the conveying belt 16, and fixed by heat and pressure, whereby a full-color print sequence is ended to form a desired full-color print image.
Then, the fixing operation of the fixing unit 18 is performed at a speed slower than a main process speed of 160 mm/sec, or at 90 mm/sec. This is because a sufficient amount of heat must be given to the toner when the unfixed image having two to four toner layers laminated is fused and color mixed, as will be described later, wherein the fixing speed slower than the main speed will provide a greater amount of heat to the toner.
The fixing unit 18 has a fixing roller 29, a pressure roller 30 for pressing on the fixing roller 29, and oil application means 0 for applying silicone oil to the fixing roller 29.
In such full-color image forming apparatus, the amount of applying silicone oil is large because toner has the great affinity and is liable to be offset.
The apparatus as illustrated in FIG. 1 is involved in image formation only on the single surface of the transfer medium, therefore, the transfer medium once having passed through the fixing unit is not supplied again.
However, if the transfer medium is supplied again for copying on both surfaces, the toner face (i.e., face on which oil is applied) will be placed on the transfer drum 15. Accordingly, the transfer medium is wound around the transfer drum 15, thus oil applied on the toner or transfer medium will be transferred onto the transfer sheet, thereafter, the oil on the transfer drum will be transferred onto the photosensitive drum 19 by rotation of the transfer drum 15 from which the transfer medium has been separated.
In this way, if the oil is transferred onto the photosensitive drum 19, the toner can not be cleaned away even by cleaning means 21 on the photosensitive drum 19, or the toner may adhere onto the oil at other portions than a desired latent image formed portion, resulting in the problem that subsequent copies may produce contaminated images with excess toner attached thereto. Owing to the oil attached onto the recording medium, the conveying force of the sheet supply roller 50 will decrease, also a sheet supply failure will occur. This phenomenon is particularly remarkable when the portion in contact with the sheet supply roller is a solid image.
Also, in the both-surface color copying, the recording medium is liable to produce wrinkles in the fixing process for the second surface.
The liability of producing wrinkles in the fixing process of the second surface is greatly dependent on the first side image, and is particularly significant when the first side image is a solid image (or a near solid image on which a great amount of toner is attached) on the central portion thereof and no image (or an image on which a small amount of toner is attached) on the peripheral portion as shown in FIGS. 5A and 5B. This is because when the recording medium is heated in the fixing process of the first side, there will occur distortion due to difference in amounts of shrinkage between the solid image portion on which a great amount of toner is attached and the white portion, and then the distortion will appear as wrinkles in the fixing process of the second side.
This mechanism will be explained with FIGS. 5A and 5B. When the transfer medium 101 is a paper, the paper will shrink in different amounts depending on the amount of toner if a toner image 102 is formed and fixed in the central portion of this paper, and it takes a convex shape expanding in the central portion as shown in FIG. 5B, if the paper is cut along the axis a--a'. If this paper is formed with another toner image on the opposite surface of this paper to the toner side 102, and passed again through the fixing unit in a direction normal to the drawing, the paper passing width is restricted, with the height in the convex portion of this paper not being relieved, because the portions C and C' having no toner image are tightly carried and fed between the sticky roller faces. Therefore, if this phenomenon takes place on the longitudinally extending paper such as A3, possibly having a slight convex, paper wrinkles will arise.